Global energy confinement scaling predictions for the kinetically stabilized tandem mirror

Abstract

Transport is studied for the kinetically stabilized tandem mirror, an attractive magnetic confinement device for achieving a steady-state burning plasma. For a magnetohydrodynamic stable system, three different radial transport models with Bohm, gyro-Bohm, and electron temperature gradient (ETG) scaling are derived. As a conservative estimate, numerical coefficients in the models are taken to be consistent with tokamak and stellarator databases. The plug mirrors create an ambipolar potential that controls end losses, whereas radial losses are driven by drift wave turbulence, which lowers the electron temperature through radially trapped particle modes and ETG transport losses. The radial transport equations are analyzed, taking into account the Pastukhov energy and particle end losses. For mirror ratio Rm=9 and a large density ratio between plug and central cell regions, there is a high axial ion confinement potential ϕi∕Ti⪢1, as demonstrated in the GAMMA-10 by Cho et al. [Nucl. Fusion 45, 1650 (2005)]. Profiles and total energy confinement times are calculated for a proof-of-principle experiment (length L=7m, central cell magnetic field B=0.28T, and radius a=1m) and for a test reactor facility (L=30m, B=3T, a=1.5m). For these parameter sets, radial loss dominates the end losses except in the low temperature periphery. In the limit of negligible radial losses, ideal ignition occurs at Ti=7.6keV from the two-body power end losses. The transport suppressing rotation rate is well below the sonic value and scales similarly to biased wall rotation rates in the Large Plasma Device experiments [Horton et al., Phys. Plasmas 12, 022303 (2005)]. Simulation results show that the positive dependence of electron radial transport with increasing electron temperature stabilizes the thermal instabilities giving steady state with Ti=30–60keV and Te=50–150keV with a fusion amplification Q of order 1.5 to 5.0.